Damage detection for wind turbine rotor blades using airborne sound

When operating a wind turbine, damage of rotor blades is a serious problem. Undetected damages are likely to increase overtime, and therefore, the safety risks and economical burdens also increase. A monitoring system, which detects reliably defects in early stages, gives scope for action and is therefore a key element to avoid damage increase and to optimize the efficiency of wind turbines. One promising approach for damage detection is acoustic emission methods. Although most acoustic emission approaches use ultrasonic sound waves of the structure that require about 12 to 40 sensors to monitor one rotor blade, we propose to use the airborne sound in lower frequencies from about 500 Hz to 35 Hz with three optical microphones and present a signal model-based damage detection algorithm. The real-time algorithm uses six audio features from a spectrogram representation to detect damages and to estimate its significance. In a fatigue test of a 34-m blade, the algorithm detected the damage event and damage increasing without false detection. It was also tested with recordings inside an operating blade of a 3.4-MW wind turbine. In the recorded time period of about 1 year, the algorithm indicated no false detection. © 2020 The Authors. Structural Control and Health Monitoring published by John Wiley & Sons Lt

Progressive Damage Mechanism of Rocks Subjected to Cyclic Loading

This thesis investigates the effect of cyclic loading conditions on the strength and deformation behaviour of rock materials under uniaxial compression. Two microstructurally and mineralogically different rocks are considered in this experimental investigation. Variations in loading stress level, stress amplitude and frequency are considered to derive novel conclusions on the damage mechanism and fatigue strength. The investigation revealed the greater susceptibility of hard rocks to cyclic loading compared to soft rocks

Mapping Acoustic and Semantic Dimensions of Auditory Perception

Auditory categorisation is a function of sensory perception which allows humans to generalise across many different sounds present in the environment and classify them into behaviourally relevant categories. These categories cover not only the variance of acoustic properties of the signal but also a wide variety of sound sources. However, it is unclear to what extent the acoustic structure of sound is associated with, and conveys, different facets of semantic category information. Whether people use such data and what drives their decisions when both acoustic and semantic information about the sound is available, also remains unknown. To answer these questions, we used the existing methods broadly practised in linguistics, acoustics and cognitive science, and bridged these domains by delineating their shared space. Firstly, we took a model-free exploratory approach to examine the underlying structure and inherent patterns in our dataset. To this end, we ran principal components, clustering and multidimensional scaling analyses. At the same time, we drew sound labels’ semantic space topography based on corpus-based word embeddings vectors. We then built an LDA model predicting class membership and compared the model-free approach and model predictions with the actual taxonomy. Finally, by conducting a series of web-based behavioural experiments, we investigated whether acoustic and semantic topographies relate to perceptual judgements. This analysis pipeline showed that natural sound categories could be successfully predicted based on the acoustic information alone and that perception of natural sound categories has some acoustic grounding. Results from our studies help to recognise the role of physical sound characteristics and their meaning in the process of sound perception and give an invaluable insight into the mechanisms governing the machine-based and human classifications

A Diagnosis Feature Space for Condition Monitoring and Fault Diagnosis of Ball Bearings

The problem of fault diagnosis and condition monitoring of ball bearings is a multidisciplinary subject. It involves research subjects from diverse disciplines of mechanical engineering, electrical engineering and in particular signal processing. In the first step, one should identify the correct method of investigation. The methods of investigation for condition monitoring of ball bearings include acoustic emission measurements, temperature monitoring, electrical current monitoring, debris analysis and vibration signal analysis. In this thesis the vibration signal analysis is employed. Once the method of analysis is selected, then features sensitive to faults should be calculated from the signal. While some of the features may be useful for condition monitoring, some of the calculated features might be extra and may not be helpful. Therefore, a feature reduction module should be employed. Initially, six features are selected as a candidate for the diagnosis feature space. After analyzing the trend of the features, it was concluded that three of the features are not appropriate for fault diagnosis. In this thesis, two problem is investigated. First the problem of identifying the effects of the fault size on the vibration signal is investigated. Also the performance of the feature space is tested in distinguishing the healthy ball bearings from the defective vibration signals

Rollup subsolar array Quarterly technical report, 5 Mar. - 30 May 1969

Thermal cycling and environmental tests for solar arra

Surface response of ceramics subject to erosive wear.

Research concerning the surface of technical ceramics is carried out from the viewpoint of their mechanical performance subjected to relevant operating conditions. The selected materials are silicon nitride, zirconia and alumina. They are subjected to cavitation erosion and other wear processes. When polished ceramics are subjected to cavitation, a pseudoplastic deformation pit is discovered, as well as a delayed phase transformation or ageing of the zirconia stored at room temperature. Measured ageing times recorded are of the order of one month. These two discoveries are novel and a full explanation is explored. The creation of cracks that release the slip planes of ceramic grains allowing plastic deformation is proposed to be the main mechanism. In order to understand the relationship of the zirconia surface degradation with its oxides (yttria or magnesia), several material compositions are tested. It is concluded that the delay of phase transformation relies on the existence of partially stabilized tetragonal zirconia regardless of the oxide used to stabilize it. Part of this research consists of understanding the effects of the experimental procedure that is utilised on the behaviour of the material. An unexpected non homogeneous cavitation erosion shape is obtained when an ultrasonic horn of small diameter is used to produce the bubbles. This non homogeneous region, referred to as ring region for this research, affects the erosion pattern of the material and it differs from the typical ones obtained when following the standard test. The acoustic theory does not explain this phenomenon. Therefore, a new approach is followed. This approach consists of using fluid mechanics equations combined with analytical mechanics principles. It is concluded that the location of cavitation clusters fulfils a condition of minimum energy. On the other hand, there are important differences between this experimental set up and the set up suggested by the published standards. These differences are discussed and a criterion for cavitation erosion resistance is developed. The comparison of cavitation resistance of materials is achieved by means of surface loss as criterion instead of volume loss

Shaker table vibration testing of a microsatellite

In the satellite development process, structural testing is a means to gain confidence in analytical models and ultimately support qualification of the spacecraft for flight. Vibration testing, in particular, is motivated by the safety considerations of crew or launch personnel, the survivability of delicate hardware and electronics, and the avoidance of large stresses that cause structural fatigue or failure. The subject of this thesis is concerned with the shaker table vibration testing of a microsatellite pair designed and built by students at the Missouri University of Science and Technology in Rolla, Missouri. A finite element model (FEM) used in structural response predictions has been formulated for the satellite, and it is the goal of these tests to verify the accuracy of the model and identify any design issues that might result in mechanical or structural damage to the spacecraft or space vehicle during flight. An introduction to environmental vibration research in the space industry is presented, including a discussion of common shaker table tests and equipment, followed by an overview of the satellite test structure. The test philosophy and implementation are introduced, and the results are presented and discussed. To offer insight for future shaker table tests, this thesis concludes with a discussion of the lessons learned. Results show that the individual microsatellites withstood the shaker excitation input, and can survive the vibration environment during flight. However, significant rattling in the cup / cone interface between the two structures necessitated a redesign of the interface. Potential solutions to this failure mode are discussed --Abstract, page iii

Acoustic emission signal analysis

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.An extension of acoustic emission technology was made which permits identification of probable source mechanisms for signals emitted during the failure of metals. This was achieved through the construction of a unique instrument and the development of special computer programs. The instrument permitted wideband digital waveform recordings to be made of both acoustic emission signals generated during the failure of a specimen, as well as calibration signals derived from a helium gas jet. These recordings were then processed by the computer programs to yield power spectra insensitive to specimen geometry, thus allowing the direct comparison of acoustic emissions from different specimens. A series of experiments conducted to test the instrument and the programs resulted in the conclusion that, at the 95% confidence level, acoustic emission caused by brittle particle fracture in 7039 aluminum could be differentiated from acoustic emission caused by the discontinuous movement of a crack in 4340 steel. Detailed descriptions of acoustic emission source modeling, transducer operating principles, calibration techniques and digital signal processing provide the necessary theoretical background for the reported technology extension, while a comprehensive review of the literature of acoustic emission places the experimental work into the proper context.This study is funded by the Naval Air Development Center, Warminster, Pennsylvania